The 35-year-old ex-St Helens half-back officially retired from playing at the end of last season after being appointed as assistant coach of the Reds.
But he has been playing rugby union part-time for Preston Grasshoppers and has made himself available for the Reds after being registered on their books.
Salford head coach Phil Veivers made the move after losing scrum-half Matty Smith to Wigan but hopes he does not need to call on him, with youngster Marc Sneyd keeping the role for Friday's home game against Widnes.
"I've signed him as cover," said Veivers. "I asked him, if we needed him, would he play and he said 'yes'. It would only be in the case of an emergency."
Long, who won every honour in the domestic game during 12 years at St Helens, made his last Super League appearance for Hull in June 2011.
"Just so it's out there," Long wrote on Twitter. "I've signed for Salford and will be called upon when needed."
Meanwhile, Veivers insists the 21-year-old Sneyd will be given the chance to claim Smith's number seven jersey in the long term.
Sneyd made only his second Super League appearance of the season in Salford's 24-22 win at Hull KR last week.
"I'm going to give him as much time as he wants this year to make the number seven jersey his own," Veivers said.